1. Field of the Invention
The present invention relates to a method of measuring the positions of a plurality of fuel rods spaced apart from each other in a fuel assembly, and an apparatus for performing such measurements.
2. Technical Background
A conventional type of fuel assembly such as the one shown in FIG. 11 comprises: an upper nozzle 1; a lower nozzle 2 which is oppositely spaced apart from the upper nozzle 1; a plurality of support grids 3, having grid cells formed by straps, disposed at a predetermined interval between the upper nozzle 1 and the lower nozzle 2; control rod guide pipes 4 which are inserted through the grid cells and attached to the support grids 3, to the upper nozzle 1 and to the lower nozzle 2; and a plurality of fuel rods 5 which are inserted into the grid cells of the support grids 3, and elastically held by the springs formed inside the support grids 3. The spacing between the fuel rods 5 in this type of fuel assembly is measured by inserting a strain gage type probe between the fuel rods 5, and the contact force on the probe is taken as a measure of the spacing.
In the conventional type of fuel assembly shown in FIG. 11, the size of the opening of the grid cells for inserting the fuel rods 5 is the same as that for inserting the guide pipes 4, thus making it possible to use the technique of inserting the probe between the fuel rods 5 to measure their spacing. However, in the other type of fuel assembly shown in FIG. 2, in which the guide pipes 10 occupy the space equivalent to four fuel rods, there are regions of the fuel rods where the strain gage type probe becomes impossible to be inserted because of the interference presented by the guide pipes 10.